Method for producing ashless coal

ABSTRACT

Provided is a method for producing ashless coal, said method comprising a slurry preparation step in which a slurry is prepared by mixing coal and a solvent, the coal that is included in the slurry is dewatered, and the temperature of the slurry is increased. The slurry preparation step comprises a preparation/dewatering step and a preparation/temperature increase step. In the preparation/dewatering step, preparation of the slurry and dewatering of the coal are performed by mixing coal and a liquid solvent that is circulated during a circulation step. In the preparation/temperature increase step, the slurry is prepared and the temperature of the slurry is increased by mixing the slurry with solvent vapor that is circulated during the circulation step.

TECHNICAL FIELD

The present invention relates to a method for producing ashless coal.

BACKGROUND ART

Conventionally, there are methods for producing ashless coal (for example, Patent Document 1). A production method of ashless coal described in Patent Document 1 is as follows (see, claim 1 of the document): “A method for producing ashless coal, including a slurry preparation step of mixing a solvent and coal to prepare a slurry, an extraction step of heating the slurry obtained in the slurry preparation step . . . to extract a coal component soluble in the solvent, a separation step of separating a coal component insoluble in the solvent from the slurry obtained in the extraction step, a step of recovering the solvent from the slurry separated in the separation step, the slurry containing a coal component insoluble in the solvent to obtain ashless coal, and a step of circulating the recovered solvent to the slurry preparation step”.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent No. 4,045,229

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The production method of ashless coal described in Patent Document 1 includes, as described above, “a step of recovering the solvent from the slurry separated in the separation step, the slurry containing a coal component insoluble in the solvent to obtain ashless coal” (this step is referred to as “ashless coal acquisition step”). The solvent recovered in the ashless coal acquisition step is at a high temperature (e.g., 270° C.). On the other hand, the production method of ashless coal described in Patent Document 1 includes “an extraction step of heating the slurry obtained in the slurry preparation step . . . to extract a coal component soluble in the solvent”. The slurry fed to the extraction step is heated (preheated) before the extraction step. Therefore, it can be considered that the heat energy possessed by the solvent (high temperature-side fluid) recovered in the ashless coal acquisition step could be used as a heat source for the slurry (low temperature-side fluid) fed to the extraction step. In addition, it can be considered that heating of the low temperature-side fluid by the high temperature-side fluid could be performed in a heat exchanger.

However, the heat exchange by a heat exchanger requires providing a temperature difference between the heat exchanger inlet temperature of the high temperature-side fluid and the heat exchanger outlet temperature of the low temperature-side fluid. Therefore, in this heat exchange, part of the heat energy of the solvent (high temperature-side fluid) recovered in the ashless coal acquisition step is not transferred to the slurry (low temperature-side fluid) fed to the extraction step.

An object of the present invention is to provide a production method of ashless coal, where a heat exchange between coal and solvent and a heat exchange between slurry and solvent are efficiently performed and the heat energy generated in the production process of ashless coal can be thereby effectively utilized.

Means for Solving the Problems

The method for producing ashless coal of the present invention includes a slurry preparation step of mixing coal and a solvent to prepare a slurry, and performing a dehydration of the coal contained in the slurry and a temperature increase of the slurry, an extraction step of heating the slurry obtained in the slurry preparation step to extract a solvent-soluble component of the coal, a separation step of separating the slurry obtained in the extraction step into a solution containing the solvent-soluble component of the coal and a solid-content concentrated liquid having concentrated therein a solvent-insoluble component of the coal, an ashless coal acquisition step of evaporating and separating the solvent from the solution separated in the separation step to obtain ashless coal, and a circulation step of circulating the solvent evaporated and separated in the ashless coal acquisition step. The slurry preparation step includes a preparation and dehydration step of mixing a solvent liquid circulated in the circulation step and the coal to thereby perform the preparation of the slurry and the dehydration of the coal, and a preparation and temperature increase step of mixing a solvent vapor circulated in the circulation step and the slurry to thereby perform the preparation and the temperature increase of the slurry.

Advantage of the Invention

Due to the configuration above, the heat exchange between coal and solvent and the heat exchange between slurry and solvent are efficiently performed, and the heat energy generated in the production process of ashless coal can be thereby effectively utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic view of the ashless coal production system 1.

FIG. 2 A schematic view of the ashless coal production system 101 of Comparative Example.

MODE FOR CARRYING OUT THE INVENTION

The production method of ashless coal and the ashless coal production system 1 for performing the production method of ashless coal are described by referring to FIG. 1.

The ashless coal production system 1 is an apparatus for producing ashless coal (HPC; Hyper-coal) by removing ash from raw material coal (sometimes simply referred to as “coal”). The ashless coal production system 1 is equipped with coal/slurry processing devices 11 to 37, circulation paths 41 to 46, and on-circulation-path devices 51 to 91.

The coal/slurry processing devices 11 to 37 are devices for processing coal and a slurry (described later). The coal/slurry processing devices 11 to 37 have a coal feed line 11 and a vapor discharge unit 13. Furthermore, the coal/slurry processing devices 11 to 37 have a slurry preparation device 20, a preheater 31, an extraction tank 33, a separation unit 35, and a solvent recovery unit 37, in the order from the upstream side in the production process of ashless coal.

The coal feed line 11 feeds coal to the slurry preparation device 20 (coal feeding step). The coal feed line 11 feeds coal from a feeder, etc. (not shown) to a preparation and dehydration tank 21 (described later) of the slurry preparation device 20. This coal is, for example, bituminous coal or low-grade coal (brown coal or subbituminous coal). The bituminous coal is higher in the extraction rate (the proportion of soluble coal component extracted with a solvent) than the low-grade coal. The low-grade coal is more inexpensive than bituminous coal.

In the vapor discharge unit 13, a purge gas is flowed into the coal feed line 11 to thereby discharge the vapor produced in a preparation and dehydration tank 21 (described later) from the coal feed line 11 (vapor discharging step). The purge gas is a gas being a gaseous state in the coal feed line 11 and is, for example, nitrogen (purge nitrogen). The vapor discharge unit 13 is provided to reduce the problem of clogging of the coal feed line 11. The “problem of clogging” above may be caused as follows. Vapor is generated as a result of heating and dehydration of coal in a preparation and dehydration tank 21 (described later). The majority of the vapor is water vapor, and part of the vapor is a solvent vapor (solvent in gaseous state). When this vapor enters the coal feed line 11, the vapor is cooled and forms a condensate, and the condensate adheres to the inner surface of the coal feed line 11. When coal adheres to the condensate, the coal clogs the coal feed line 11 (the passage is narrowed or the passage is completely clogged). In this way, the “problem of clogging” may be caused. The vapor discharge unit 13 has a valve 13 a and a purge gas feed unit 13 b.

The valve 13 a is disposed on the coal feed line 11. The flow of substances (coal, vapor, and purge gas) passing through the coal feed line 11 is controlled by the opening/closing of the valve 13 a. A plurality (for example, two) of valves 13 a are preferably disposed in series on the coal feed line 11. In the case of disposing a plurality of valves 13 a on the coal feed line 11, the vapor can be more effectively prevented from entering the coal feed line 11, compared with a case where only one is disposed. The valve 13 a is, for example, a rotary valve.

The purge gas feed unit 13 b feeds a purge gas into the coal feed line 11 (purge gas feeding step). In the case where two valves 13 a are disposed in series on the coal feed line 11, the purge gas feed unit 13 b feeds a purge gas between two valves 13 a.

In the slurry preparation device 20, coal and a solvent are mixed to prepare a slurry (coal-solvent slurry) and the dehydration and temperature increase of slurry are performed (slurry preparation step).

The solvent fed to the slurry preparation device 20 is one which dissolves coal. The solvent is preferably one ensuring that the proportion (extraction rate) of soluble coal component extracted in the extraction tank 33 is high. In view of extraction rate, the solvent is preferably stable even in a heated state and preferably has a high dissolving power for coal (an excellent affinity for coal). This solvent is preferably one providing a high solvent recovery rate in a solvent recovery unit 37. In view of the solvent recovery rate, it is preferred that the solvent is readily recoverable by distillation or other methods. In view of pressure reduction in the extraction tank 33 and the separation unit 35 and the extraction rate in the extraction tank 33, etc., the boiling point of the solvent is, for example, preferably from 180 to 300° C. and more preferably from 230 to 280° C. The solvent is, for example, a coal derivative. The solvent is obtained by mainly refining a carbonization product of coal. The solvent is, for example, a solvent containing an aromatic compound (aromatic solvent). The main component of the solvent is a bicyclic aromatic. The bicyclic aromatic includes, for example, naphthalene, methylnaphthalene, dimethylnaphthalene, and trimethylnaphthalene. Other components of the solvent include, for example, naphthalenes, anthracenes and fluorenes, each having an aliphatic side chain, and alkylbenzenes formed by adding biphenyl or a long-chain aliphatic side chain thereto. Specifically, the solvent is, for example, a methylnaphthalene oil or a naphthalene oil. The methylnaphthalene oil or naphthalene oil is a distillation oil of a byproduct oil in carbonization of coal for producing coke. The slurry preparation device 20 has a preparation and dehydration tank 21 and a preparation and temperature increase device 23.

In the preparation and dehydration tank 21, a solvent liquid (solvent in a liquid state) and coal are mixed to thereby perform the preparation of slurry and the dehydration of coal (preparation and dehydration step). The solvent liquid fed to the preparation and dehydration tank 21 is a solvent liquid circulated through part of the circulation paths 41 to 46. The coal fed to the preparation and dehydration tank 21 is fed by way of the coal feed line 11. The mixing of solvent liquid and coal in the preparation and dehydration tank 21 is performed, for example, by charging coal into the solvent liquid in the preparation and dehydration tank 21. A slurry is prepared by the mixing of solvent liquid and coal in the preparation and dehydration tank 21. The S/C (Slurry/Coal; a ratio of the mass of slurry to the mass of coal in a dry state) of the slurry prepared in the preparation and dehydration tank 21 is, for example, about 2.0.

In the preparation and dehydration tank 21, the dehydration of coal is performed as follows. In the preparation and dehydration tank 21, the solvent liquid is brought into direct contact with coal by the mixing of solvent liquid and coal. In the preparation and dehydration tank 21, due to the direct contact above, heat exchange is directly performed between the solvent liquid and coal. By this heat exchange in the preparation and dehydration tank 21, the temperature of the coal is increased and the water (water contained by coal) in the coal is evaporated. The temperature of the solvent liquid fed to the preparation and dehydration tank 21 is equal to or more than the temperature necessary to perform the dehydration and is less than the boiling point of the solvent. The temperature of the solvent liquid fed to the preparation and dehydration tank 21 is, for example, 230° C. or more and preferably 235° C. or more, and, for example, 240° C. or less. In the example illustrated in FIG. 1, the temperature of the solvent liquid fed to the preparation and dehydration tank 21 is 237° C. (hereinafter, refer to FIG. 1 as to specific examples of the temperature).

In the preparation and temperature increase device 23, a solvent vapor and the slurry are mixed to thereby perform the preparation and temperature increase of slurry (preparation and temperature increase step). In the preparation and temperature increase device 23, the concentration of the slurry is adjusted to be an inlet concentration of the extraction tank 33. The inlet concentration of the extraction tank 33 is set in advance. A device for adjusting the slurry concentration need not be provided between the preparation and temperature increase device 23 and the extraction tank 33, and the ashless coal production system 1 does not have such a device. The S/C of the slurry prepared in the preparation and temperature increase device 23 is, for example, about 4.0. In the preparation and temperature increase device 23, the temperature of the slurry is increased to an inlet temperature of a device (referred to as “post-slurry-preparation device”) to which the slurry is fed following the slurry preparation device 20. The inlet temperature of the “post-slurry-preparation device” is set in advance. Specifically, the “post-slurry-preparation device” is the preheater 31 and, in the case of not providing a preheater 31, it is the extraction tank 33. A device for adjusting the slurry temperature need not be provided between the preparation and temperature increase device 23 and the “post-slurry-preparation device”, and the ashless coal production system 1 does not have such a device. The preparation and temperature increase device 23 has a venturi scrubber 23 a and a preparation and temperature increase tank 23 b.

The venturi scrubber 23 a mixes a solvent vapor and the slurry (first preparation and temperature increase step). The solvent vapor fed to the venturi scrubber 23 a is the solvent vapor circulated through part of the circulation paths 41 to 46 (details are described later). The slurry fed to the venturi scrubber 23 a is fed from the preparation and dehydration tank 21. The slurry fed to the venturi scrubber 23 a is the slurry after preparation and dehydration thereof have been performed in the preparation and dehydration tank 21. In the venturi scrubber 23 a, the slurry is brought into direct contact with the solvent vapor by the mixing of slurry and solvent vapor. In the venturi scrubber 23 a, due to the direct contact above, heat exchange is directly performed between the solvent vapor and the slurry. In the venturi scrubber 23 a, the slurry is heated by utilizing latent heat of the solvent vapor (by utilizing heat generated at the time of condensation of the solvent vapor). In the case where dehydration of coal in the preparation and dehydration tank 21 is not completed, the venturi scrubber 23 a heats the slurry to thereby perform the dehydration of coal in the slurry. The venturi scrubber 23 a forms the slurry into fine particle state and mixes the fine particulate slurry and the solvent vapor. In the venturi scrubber 23 a, the fine particulate slurry and the solvent vapor are mixed by increasing the flow rate of the fine particulate slurry and the solvent vapor and thereby producing a shear force on the fine particulate slurry and the solvent vapor. Here, in place of or in addition to the venturi scrubber 23 a, a unit (first preparation and temperature increase unit) for mixing the solvent vapor and the slurry, other than a venturi scrubber, may also be used. The “unit other than a venturi scrubber” includes, for example, a static mixer. In the static mixer, the fine particulate slurry and the solvent vapor are stirred and mixed by an element (a member of twisted plate shape or a member of screw shape) disposed inside a tube.

In the preparation and temperature increase tank 23 b, the mixture mixed in the venturi scrubber 23 a is further mixed (second preparation and temperature increase step). In the preparation and temperature increase tank 23 b, due to the mixing above, heat is further exchanged between the slurry and the solvent. The interior of the preparation and temperature increase tank 23 b is pressurized so that the vaporization of solvent (solvent vapor loss) can be suppressed, and it is pressurized, for example, to 50 kPaG.

In the preheater 31, the slurry prepared in the preparation and temperature increase device 23 (slurry preparation device 20) is previously heated before being fed to the extraction tank 33 (preheating step). The preheater 31 may not be used.

In the extraction tank 33, the slurry obtained in the slurry preparation device 20 is heated to extract a coal component soluble in the solvent (solvent-soluble component) (extraction step). In the extraction tank 33, an organic component in the coal is extracted. Details of this extraction are as follows. The slurry fed to the extraction tank 33 is heated and kept at a predetermined temperature (described later) under stirring by a stirrer provided in the extraction tank 33. In this manner, a solvent-soluble component is extracted from the slurry. However, not only a solvent-soluble component but also a component insoluble in the solvent (solvent-insoluble component) (for example, ash) are contained in the extract.

The slurry heating temperature in the extraction tank 33 is a temperature allowing the solvent-soluble component to be dissolved in the solvent. Specifically, the slurry heating temperature is, for example, 300° C. or more, and preferably 360° C. or more. The slurry heating temperature is, for example, 420° C. or less, and preferably 400° C. or less. If the slurry heating temperature is less than 300° C., it is not enough to weaken the bond between coal molecules and therefore, the amount of the solvent-soluble component dissolved in the solvent decreases. If the slurry heating temperature exceeds 420° C., the coal undergoes a vigorous pyrolysis reaction and recombination of produced pyrolysis radicals occurs, and as a result, the extraction rate of the solvent-soluble component decreases.

The extraction conducted in the extraction tank 33 is preferably performed in the presence of an inert gas (preferably, for example, nitrogen that is inexpensive). In order to perform this extraction, the solvent must be confined to a liquid phase (the solvent must be prevented from volatilizing). For confining the solvent to the liquid phase, the pressure (pressure applied to the solvent and the slurry, operation pressure) in the extraction tank 33 needs to be higher than the vapor pressure of the solvent. The pressure in the extraction tank 33 is preferably from 1.0 to 2.0 MPa, though this may vary depending on the temperature during extraction or the vapor pressure of the solvent used.

In the separation unit 35, the slurry obtained in the extraction tank 33 is separated into a solution (solution part, supernatant, overflow) containing a solvent-soluble component of the coal and a solid-content concentrated liquid (underflow) having concentrated therein a solvent-insoluble component of the coal (separation step). The method for this separation is, for example, a gravitational settling method, a filtration method or a centrifugal separation method. The gravitational settling method is a method of holding the slurry in the tank and settling the solvent-insoluble component by utilizing gravity, thereby achieving separation into a solution and a solid-content concentrated liquid. In the following, a case where separation in the separation unit 35 is performed by the gravitational settling method is described. The interior of the separation unit 35 is kept warm (or heated) and pressurized. This warm keeping (or heating) and pressurization are performed so as to prevent reprecipitation of the solvent-soluble component eluted from the coal. The temperature in the separation unit 35 is, for example, from 300 to 380° C. The pressure in the separation unit 35 is, for example, from 1.0 to 3.0 MPa. The separation unit 35 is, for example, a two-stage type (the number of gravitational settling tanks is 2). The two-stage type separation unit 35 has a first gravitational settling tank 35 a and a second gravitational settling tank 35 b. The separation unit 35 may be a one-stage type (the number of gravitational settling tanks is 1). Here, it is ideal that the separation unit 35 completely separates a supernatant and a solid-content concentrated liquid, but a solid content (coal component insoluble in the solvent) may get mixed in with part of the supernatant, or the supernatant may get mixed in with part of the solid-content concentrated liquid.

In the solvent recovery unit 37, the solvent is recovered from the solution separated in the separation unit 35. The solvent recovery unit 37 is a unit for obtaining ashless coal or byproduct coal (described later) from the solution separated in the separation unit 35. The solvent recovery unit 37 has a first solvent recovery unit 37 a and a second solvent recovery unit 37 b.

The first solvent recovery unit 37 a is a unit for obtaining ashless coal (HPC) (a unit for performing the ashless coal acquisition step) by evaporating and separating the solvent from the solution separated in the separation unit 35. The ashless coal is coal containing absolutely no water and containing substantially no ash. The amount of ash contained in the ashless coal is 5 wt % or less and preferably 3 wt % or less. The ashless coal has a higher heating value, and better ignitability and burnout performance than those of a raw material coal and therefore, is used, for example, as a high-efficient fuel for a boiler, etc. The ashless coal is higher in the fluidity (plastic properties) than a raw material coal and is used, for example, as a raw material or part of a raw material (blended coal) of coke for iron making.

The method for the evaporation and separation of solvent performed in the first solvent recovery unit 37 a includes, for example, a distillation method and an evaporation method. The evaporation method includes, for example, a spray dry method. The distillation method includes, for example, a flash distillation method and a thin-film distillation method. The first solvent recovery unit 37 a is, for example, a flash tank (flasher) for performing a flash distillation method. Alternatively, the first solvent recovery unit 37 a is, for example, a thin-film distillation tank for performing a thin-film distillation method. In addition, the first solvent recovery unit 37 a is, for example, a unit having a flash tank and a thin-film distillation tank (disposed, for example, on the downstream side of the flash tank).

(Flash Method)

The evaporation and separation of solvent by the flash method is performed as follows. The pressure in the flash tank is adjusted to a low pressure (for example, 70 kPaG) compared with that in the separation unit 35. Then, the solution separated in the separation unit 35 spouts into the flash tank. As a result, the solvent in the solution is evaporated and separated from the solution.

(Thin-Film Distillation Method)

The evaporation and separation of solvent by the thin-film distillation method is performed as follows. The solution separated in the separation unit 35 is introduced into the thin-film distillation tank. Thereafter, a scraper (also called “wiper”) housed in the thin-film distillation tank forms a thin film of the distillation object (the solution separated in the distillation unit 35) on the inner wall of the thin-film distillation tank, whereby continuous distillation is performed. The pressure in the thin-film distillation tank is, for example, 0.1 MPaG. In order to allow for appropriate evaporation of the solvent in the thin-film distillation tank, the wall of the thin-film distillation tank is heated. The heating of wall of the thin-film distillation tank is performed, for example, by hot oil or performed, for example, by an electric heater. In the case of performing the heating of wall of the thin-film distillation tank by hot oil, a jacket (coating) is provided on the inner and outer sides (or, for example, on either one of the inner side and the outer side) of wall of the thin-film distillation tank. Hot oil is flowed through the jacket. As a result, the wall of the thin-film distillation tank is heated. The heating of wall of the thin-film distillation tank is required, for example, in the following case. The first solvent recovery unit 37 a sometimes has a flash distillation tank and a thin-film distillation tank on the downstream side of the flash distillation tank. In this case, the solution temperature drops due to evaporation in the flash tank. Therefore, the heating of wall of the thin-film distillation tank is performed so that the solvent can be appropriately evaporated in the thin-film distillation tank.

The second solvent recovery unit 37 b is a unit for obtaining byproduct coal (RC: Residue coal) (also called residual coal) (a unit for performing the byproduct coal acquisition step) by evaporating and separating the solvent from the solid-content concentrated liquid separated in the separation unit 35. The byproduct coal is coal having concentrated therein a solvent-insoluble component (for example, ash) and is used, for example, as part of the blended coal that is a raw material of coke. The method for the evaporation and separation of solvent in the second solvent recovery unit 37 b includes a distillation method and an evaporation method, similarly to the method for the evaporation and separation of solvent in the first solvent recovery unit 37 a. The second solvent recovery unit 37 b may not be provided.

In the circulation paths 41 to 46, the solvent evaporated and separated in the solvent recovery unit 37, etc. is circulated (circulation step). The circulation paths 41 to 46 are flow channels (pipings) for recycling the solvent. The circulation paths 41 to 46 include a first circulation path 41, a second circulation path 42, a third circulation path 43, a fourth circulation path 44, a fifth circulation path 45, and a sixth circulation path 46.

In the first circulation path 41, the solvent evaporated and separated in the first solvent recovery unit 37 a is circulated to the preparation and dehydration tank 21 (first circulation step). The first circulation path 41 introduces the solvent taken out from the top of the first solvent recovery unit 37 a into the preparation and dehydration tank 21.

In the second circulation path 42, the solvent evaporated and separated in the first solvent recovery unit 37 a is circulated to the preparation and temperature increase device 23 (second circulation step). In the second circulation path 42, the solvent evaporated and separated in the first solvent recovery unit 37 a is circulated to the venturi scrubber 23 a. In the second circulation path 42, the solvent evaporated and separated in the first solvent recovery unit 37 a is directly (without heat recovery or temperature increase) circulated to the venturi scrubber 23 a. The sum of the piping pressure drop (for example, 20 kPaG) in the second circulation path 42 and the operation pressure (for example, 50 kPaG) in the venturi scrubber 23 a is equal to the operation pressure (for example, 70 kPaG) in the first solvent recovery unit 37 a. Therefore, the operation pressure in the first solvent recovery unit 37 a is set based on the sum of the operation pressure in the venturi scrubber 23 a and the piping pressure drop in the second circulation path 42.

In the third circulation path 43, the solvent evaporated and separated in the first solvent recovery unit 37 a is circulated to the separation unit 35 (third circulation step). In the third circulation path 43, the solvent evaporated and separated in the first solvent recovery unit 37 a is circulated to the second gravitational settling tank 35 b. By feeding the solvent (a high-temperature solvent concentrate) to the second gravitational settling tank 35 b through the third circulation path 43, the supply of solvent necessary for the second gravitational settling tank 35 b is provided.

In the fourth circulation path 44, the solvent evaporated and separated in the second solvent recovery unit 37 b is circulated to the preparation and dehydration tank 21 (fourth circulation step). The fourth circulation path 44 introduces the solvent taken out from the top of the second solvent recovery unit 37 b into the preparation and dehydration tank 21. In the vapor introduced into the fourth circulation path 44 from the second solvent recovery unit 37 b, not only the solvent but also nitrogen are contained.

In the fifth circulation path 45, the vapor generated in the slurry preparation device 20 is circulated to the preparation and dehydration tank 21 (fifth circulation step). The vapor generated in the slurry preparation device 20 contains the solvent and water and contains water in a larger ratio than the solvent. In the fifth circulation path 45, vapor generated in the preparation and dehydration tank 21 is circulated to the preparation and dehydration tank 21. In the firth circulation path 45, vapor generated in the preparation and temperature increase tank 23 b is circulated to the preparation and dehydration tank 21. The portion “A” depicted on the upper side of the slurry preparation device 20 in FIG. 1 is connected to the portion “A” depicted on the left side of a cooler 81 in the same figure.

In the sixth circulation path 46, the solvent vapor generated in the extraction tank 33 is circulated to the preparation and dehydration tank 21 (sixth circulation step).

The on-circulation-path devices 51 to 91 are devices disposed on the circulation paths 41 to 46 (excluding the second circulation path 42). The on-circulation-path devices 51 to 91 include the following devices. The device disposed on the first circulation path 41 includes a recovered solvent tank 51. The device disposed on the third circulation path 43 includes, in the order from the upstream side, an exhaust heat recovery boiler 61, a heat exchanger 63 and a preheater 65. The device disposed on the fourth circulation path 44 includes, in the order from the upstream side, a bag filter 71, an exhaust heat recovery boiler 73, a cooler 75, a heat exchanger 77, and a recovered solvent tank 51. The device disposed on the fifth circulation path 45 includes, in the order from the upstream side, a cooler 81, an oil/water separation tank 83, a heat exchanger 77, and a recovered solvent tank 51. The device disposed on the sixth circulation path 46 includes, in the order from the upstream side, an oil heater 91, a heat exchanger 63, a heat exchanger 77, an oil/water separation tank 83, a heat exchanger 77, and a recovered solvent tank 51.

In the recovered solvent tank 51, a solvent liquid for feeding to the preparation and dehydration tank 21 is prepared (solvent liquid preparation step). The solvent fed to the recovered solvent tank 51 is the solvent (solvent vapor) passing through the first circulation path 41 and is more specifically the solvent vapor evaporated and separated in the first solvent recovery unit 37 a. The solvent vapor evaporated and separated in the first solvent recovery unit 37 a is directly (without heat recovery or temperature increase) fed to the recovered solvent tank 51. In addition, the solvent fed to the recovered solvent tank 51 is the solvent flowing through the fourth circulation path 44, the fifth circulation path 45 and the sixth circulation path 46 and is more specifically the solution liquid after being subjected to heat exchange in the heat exchanger 77 (described later).

In the exhaust heat recovery boiler 61, heat of the solvent flowing through the third circulation path 43 is recovered (third circulation path exhaust heat recovery step). The solvent fed to the exhaust heat recovery boiler 61 is the solvent (solvent vapor) passing through the third circulation path 43 and is more specifically the solvent vapor evaporated and separated in the first solvent recovery unit 37 a. The solvent vapor evaporated and separated in the first solvent recovery unit 37 a is directly fed to the exhaust heat recovery boiler 61. In the exhaust heat recovery boiler 61, saturated vapor (steam) is produced by utilizing the heat energy of the solvent fed to the exhaust heat recovery boiler 61. In the exhaust heat recovery boiler 61, the temperature of the solvent vapor fed to the exhaust heat recovery boiler 61 is lowered to condensate the solvent vapor. In the exhaust heat recovery boiler 61, saturated vapor of, for example, 2.2 MPaG is produced, for example, at 19.30 t/h. The exhaust heat recovery boiler 61 may be replaced by an exhaust heat recovery unit other than a boiler. As to the replacement with an exhaust heat recovery unit other than a boiler, the same applies to the later-described exhaust heat recovery boiler 73, etc. The exhaust heat recovery unit other than a boiler includes, for example, a unit for heating hot oil (see, the oil heater 91 described later).

In the heat exchanger 63, the temperature of the solvent flowing through the third circulation path 43 is increased (third circulation path temperature increase step). The low temperature-side fluid (fluid to be temperature-increased) fed to the heat exchanger 63 is the solvent flowing through the third circulation path 43 and is more specifically the solvent liquid after being subjected to heat exchange in the exhaust heat recovery boiler 61. The high temperature-side fluid (fluid to increase temperature) fed to the heat exchanger 63 is the solvent flowing through the sixth circulation path 46 and is more specifically the solvent (solvent vapor) after being subjected to heat recovery in the oil heater 91 (described later).

In the preheater 65, the solvent flowing through the third circulation path 43 is previously heated before being fed to the separation unit 35 (third circulation path preheating step). The solvent fed to the preheater 65 is the solvent (solvent liquid) after being subjected to heat exchange in the heat exchanger 63. In the preheater 65, the temperature of the solvent is increased to a required temperature in the separation unit 35 (second gravitational settling tank 35 b).

In the bag filter 71, the solvent, etc. flowing through the fourth circulation path 44 is filtered (filtration step). The solvent fed to the bag filter 71 is the solvent (solvent vapor) evaporated and separated in the second solvent recovery unit 37 b.

In the exhaust heat recovery boiler 73, heat of the solvent flowing through the fourth circulation path 44 is recovered (fourth circulation path exhaust heat recovery step). The solvent fed to the exhaust heat recovery boiler 73 is the solvent (solvent vapor) having filtered in the bag filter 71. In the exhaust heat recovery boiler 73, saturated vapor is produced by utilizing the heat energy of the solvent. In the exhaust heat recovery boiler 73, saturated vapor of, for example, 0.70 MPaG is produced, for example, at 6.03 t/h.

In the cooler 75, the solvent flowing through the fourth circulation path 44 is cooled (fourth circulation path cooling step). In the cooler 75, the solvent is cooled by using, for example, cooling water. The solvent fed to the cooler 75 is the solvent (solvent vapor) having heat-recovered in the exhaust heat recovery boiler 73. In the cooler 75, the solvent vapor fed to the cooler 75 is cooled and condensed.

In the heat exchanger 77, the temperature of the solvent flowing through the fourth circulation path 44 is increased (fourth circulation path temperature increase step). In the heat exchanger 77, the temperature of the solvent flowing through the fifth circulation path 45 is increased (fifth circulation path temperature increase step). In the heat exchanger 77, the temperature of the solvent flowing through the sixth circulation path 46 is increased (sixth circulation path temperature increase step). The low temperature-side fluid fed to the heat exchanger 77 is the solvent flowing through the fourth circulation path 44 and is more specifically the solvent (solvent liquid) after being subjected to cooling in the cooler 75. The low temperature-side fluid fed to the heat exchanger 77 is the solvent flowing through the fifth circulation path 45 and sixth circulation path 46 and is more specifically the solvent (solvent liquid) after being subjected to oil/water separation in an oil/water separation tank 83 (described later). The high temperature-side fluid fed to the heat exchanger 77 is the solvent flowing through the sixth circulation path 46 and is more specifically the solvent (solvent vapor) after being subjected to heat exchange in the heat exchanger 63 but before oil/water separation in an oil/water separation tank 83.

In the cooler 81, the vapor (as described above, the vapor containing the solvent and water) flowing through the fifth circulation path 45 is cooled (fifth circulation path cooling step). In the cooler 81, the vapor is cooled by using, for example, cooling water. In the cooler 81, the vapor is cooled and condensed.

In the oil/water separation tank 83, the solvent (oil) and water are separated from the fluid flowing through the fifth circulation path 45, etc. (oil/water separation step). The fluid fed to the oil/water separation tank 83 is the fluid flowing through the fifth circulation path 45 and is more specifically the liquid after being subjected to cooling in the cooler 81. The fluid fed to the oil/water separation tank 83 is the fluid flowing through the sixth circulation path 46 and is more specifically the solvent (solvent liquid) after heat exchange in the heat exchanger 77. The water separated in the oil/water separation tank 83 is discharged as waste water (WW) from the oil/water separation tank 83.

In the oil heater 91, the temperature of hot oil is increased by utilizing the heat energy of the solvent (solvent vapor) flowing through the sixth circulation path 46 (oil temperature increase step). The solvent fed to the oil heater 91 is the solvent vapor generated in the extraction tank 33. The hot oil temperature-increased in the oil heater 91 is utilized as a heat source in other steps. The hot oil is utilized, for example, as a heat source in the solvent recovery unit 37. The hot oil is used, for example, for the heating of wall of the thin-film distillation tank of the solvent recovery unit 37 as described above. The oil heater 91 may be replaced by an exhaust heat recovery unit (e.g., boiler) other than a unit for increasing the temperature of hot oil.

(Ashless Coal Production System 101 of Comparative Example)

In order to perform a comparison in the later-described “Comparison of Utility Amount”, etc., the ashless coal production system 101 of Comparative Example illustrated in FIG. 2 is described. The difference between the ashless coal production system 101 and the ashless coal production system 1 (see, FIG. 1) (the difference affecting the comparison of utility amount) is as in the following [Difference a] to [Difference e]. As to the configurations shared in common by the ashless coal production system 101 and the ashless coal production system 1 (see, FIG. 1), the same symbols are used.

[Difference a]

The ashless coal production system 1 illustrated in FIG. 1 is equipped with a preparation and dehydration tank 21 and a preparation and temperature increase device 23. Instead, the ashless coal production system 101 illustrated in FIG. 2 is equipped with, in the order from the upstream side, a slurry preparation tank 121, a dehydration tank 122 and a temperature increase tank 123. In the slurry preparation tank 121, coal and a solvent are mixed to prepare a slurry. In the dehydration tank 122, coal in the slurry prepared in the slurry preparation tank 121 is dehydrated. In the temperature increase tank 123, the temperature of the slurry after being subjected to dehydration in the dehydration tank 122 is increased.

[Difference b]

The ashless coal production system 1 illustrated in FIG. 1 is equipped with a vapor discharge unit 13 on the coal feed line 11, but in the ashless coal production system 101 illustrated in FIG. 2, a vapor discharge unit 13 (see, FIG. 1) is not provided. Therefore, the “problem of clogging” (described above) of the coal feed line 11 cannot be reduced by a vapor discharge unit 13. In order to prevent water in the coal from evaporating in the slurry preparation tank 121, the solvent fed to the slurry preparation tank 121 is cooled (for example, to 107° C.). Specifically, the solvent is cooled by the configurations or steps in the following [Difference c] to [Difference e].

[Difference c]

The ashless coal production system 101 is equipped with a first circulation path 141. The first circulation path 141 is a flow channel corresponding to the first circulation path 41 and the second circulation path 42 of the ashless coal production system 1 illustrated in FIG. 1. The first circulation path 141 illustrated in FIG. 2 is a flow channel for feeding the solvent vapor evaporated and separated in the first solvent recovery unit 37 a to the slurry preparation tank 121. On the first circulation path 141, the dehydration tank 122 and the temperature increase tank 123 are disposed in the order from the upstream side. In the first circulation path 141, the solvent (solvent vapor) evaporated and separated in the first solvent recovery unit 37 a is flowed to the dehydration tank 122 and the temperature increase tank 123. In this manner, heat exchange is indirectly performed between the solvent flowing through the first circulation path 141, and the slurry in the dehydration tank 122 and the temperature increase tank 123. That is, the heat energy of the solvent vapor evaporated and separated in the first solvent recovery unit 37 a is used as a heat source for the dehydration and temperature increase of the slurry in the dehydration tank 122 and the temperature increase tank 123.

[Difference d]

The ashless coal production system 101 is equipped with an exhaust heat recovery boiler 153 and a cooler 155. The exhaust heat recovery boiler 153 and the cooler 155 are disposed on the first circulation path 141. In the exhaust heat recovery boiler 153, saturated vapor is produced by utilizing the heat energy of the solvent (solvent liquid) after being subjected to heat exchange in the temperature increase tank 123. In the exhaust heat recovery boiler 153, saturated vapor of 0.50 MPaG is produced at 8.18 t/h. In the cooler 155, the solvent (solvent liquid) after being subjected to heat recovery in the exhaust heat recovery boiler 153 is cooled by using cooling water.

[Difference e]

The ashless coal production system 101 is equipped with an exhaust heat recovery boiler 193 and a cooler 195. The exhaust heat recovery boiler 193 and the cooler 195 are disposed on the sixth circulation path 46. In the case of the ashless coal production system 101, the sixth circulation path 46 circulates the vapor generated in the extraction tank 33 to the slurry preparation tank 121. In the exhaust heat recovery boiler 193, saturated vapor is produced by utilizing the heat energy of the solvent (solvent vapor) after the temperature increase of hot oil in the oil heater 91. In the exhaust heat recovery boiler 193, saturated vapor of 0.5 MPaG is produced at 1.72 t/h. In the cooler 195, the solvent (solvent liquid) after being subjected to heat recovery in the exhaust heat recovery boiler 193 is cooled by using cooling water. In the exhaust heat recovery boiler 73 of the ashless coal production system 101, saturated vapor of 0.50 MPaG is produced at 6.88 t/h.

(Comparison of Utility Amount)

The utility amount in the ashless coal production method of this embodiment (in the case of using the ashless coal production system 1 illustrated in FIG. 1), relative to the ashless coal production method of Comparative Example (in the case of using the ashless coal production system 101), is shown below.

-   -   Amount of saturated vapor generated: increase of about 50%     -   Amount of cooling water used: decrease of about 30 wt %

(Amount of Saturated Vapor Generated)

In the ashless coal production system 1 illustrated in FIG. 1, the amount of saturated vapor generated is the total amount of saturated vapors produced in the exhaust heat recovery boiler 61 and the exhaust heat recovery boiler 73. In the ashless coal production system 101 illustrated in FIG. 2, the amount of saturated vapor generated is the total amount of saturated vapors produced in the exhaust heat recovery boiler 153, the exhaust heat recovery boiler 193 and the exhaust heat recovery boiler 73. It can be seen from the comparison result above, when the ashless coal production system 1 illustrated in FIG. 1 is used, the amount of vapor recoverable in the exhaust heat recovery boiler can be increased, compared with Comparative Example.

(Amount of Cooling Water Used)

In the ashless coal production system 1, the amount of cooling water used is the total amount of cooling waters used in the cooler 75 and the cooler 81. In the ashless coal production system 101 illustrated in FIG. 2, the amount of cooling water used is the total amount of cooling waters used in the cooler 155, the cooler 195 and the cooler 75. It can be seen from the comparison result above, when the ashless coal production system 1 illustrated in FIG. 1 is used, the amount of cooling water used in the cooler can be decreased, compared with Comparative Example. As a result, the running cost of the ashless coal production system 1 can be reduced, compared with Comparative Example.

(Effects)

The effects by the ashless coal production method of this embodiment are described below. In the following, the device used for performing each step (the device corresponding to each step) is indicated in parentheses after the name of step.

(Effect 1)

The ashless coal production method (ashless coal production system 1) includes a slurry preparation step (slurry preparation device 20), an extraction step (extraction tank 33), a separation step (separation unit 35), an ashless coal acquisition step (first solvent recovery unit 37 a), and a circulation step (first circulation path 41 and second circulation path 42). The slurry preparation step (slurry preparation device 20) is a step of mixing coal and a solvent to prepare a slurry, and performing dehydration and temperature increase of slurry. The extraction step (extraction tank 33) is a step of heating the slurry obtained in the slurry preparation step (slurry preparation device 20) to extract a solvent-soluble component of the coal. The separation step (separation unit 35) is a step of separating the slurry obtained in the extraction step (extraction tank 33) into a solution containing the solvent-soluble component of the coal and a solid-content concentrated liquid having concentrated therein a solvent-insoluble coal component. The ashless coal acquisition step (first solvent recovery unit 37 a) is a step of evaporating and separating the solvent from the solution separated in the separation step (separation unit 35) to obtain ashless coal. The circulation step (first circulation path 41 and second circulation path 42) is a step of circulating the solvent evaporated and separated in the ashless coal acquisition step (first solvent recovery unit 37 a). The slurry preparation step (slurry preparation device 20) includes a preparation and dehydration step (preparation and dehydration tank 21) and a preparation and temperature increase step (preparation and temperature increase device 23).

[Configuration 1-1]

The preparation and dehydration step (preparation and dehydration tank 21) is a step of mixing a solvent liquid circulated in the circulation step (first circulation path 41) and the coal to thereby perform the preparation of the slurry and the dehydration of the coal.

[Configuration 1-2]

The preparation and temperature increase step (preparation and temperature increase device 23) is a step of mixing a solvent vapor circulated in the circulation step (second circulation path 42) and the slurry to thereby perform the preparation and the temperature increase of the slurry.

In the preparation and dehydration step (preparation and dehydration tank 21) of [Configuration 1-1], mixing of solvent and coal is performed. Due to this mixing, the solvent is brought into direct contact with coal, and heat exchange is thereby directly performed between the solvent and coal. In addition, in the preparation and temperature increase step (preparation and temperature increase device 23) of [Configuration 1-2], mixing of solvent and slurry is performed. Due to this mixing, the solvent is brought into direct contact with the slurry, and heat exchange is thereby directly performed between the solvent and the slurry. These direct heat exchanges are more efficient, compared with indirect heat exchange (for example, heat exchange using a heat exchanger). Specifically, for example, the heat exchange using a heat exchanger requires providing a temperature difference between the heat exchanger inlet temperature of the high temperature-side fluid (fluid to increase temperature) and the heat exchanger outlet temperature of the low temperature-side fluid (fluid to be temperature-increased). On the other hand, in the direct heat exchange performed in [Configuration 1-1] and [Configuration 1-2], the above-described temperature difference can be regarded as 0. Therefore, the heat exchange between coal and solvent and the heat exchange between slurry and solvent can be efficiently performed (this action is referred to as [Action 1-1]).

Furthermore, in the preparation and dehydration step (preparation and dehydration tank 21) of [Configuration 1-1], a solvent liquid (liquid) and coal (solid) are mixed. The heat exchange between solvent liquid (liquid) and coal (solid) is more efficient, compared with the heat exchange between solvent vapor (gas) and coal (solid). In addition, in the preparation and temperature increase step (preparation and temperature increase device 23) of [Configuration 1-2], a solvent vapor (gas) and a slurry (a mixture of solid and liquid) are mixed. The heat exchange between solvent vapor (gas) and slurry (a mixture of solid and liquid) is more efficient, compared with the heat exchange between solvent vapor (gas) and coal (solid). Therefore, by virtue of [Configuration 1-1] and [Configuration 1-2], the heat exchange between coal and solvent and the heat exchange between slurry and solvent can be efficiently performed (this action is referred to as [Action 1-2]). Due to [Action 1-1] and [Action 1-2], the heat energy generated in the production process of ashless coal can be effectively utilized.

(Effect 2) [Configuration 2]

In the preparation and temperature increase step (preparation and temperature increase device 23), the concentration of the slurry is adjusted to be an inlet concentration of the extraction step (extraction tank 33), which is an inlet concentration set in advance.

By virtue of [Configuration 2], the concentration of slurry need not be adjusted after the preparation and temperature increase step but before the extraction step (between the preparation and temperature increase device 23 and the extraction tank 33). Therefore, the number of devices can be reduced, compared with a case where a device for adjusting the concentration of slurry after the extraction step but before the extraction step needs to be provided. As a result, the equipment cost of the equipment (ashless coal production system 1) for performing the ashless coal production method can be reduced.

(Effect 3) [Configuration 3]

In the preparation and temperature increase step (preparation and temperature increase device 23), the temperature of slurry is increased to an inlet temperature of a step (for example, preheater 31) performed following the slurry preparation step (slurry preparation device 20), which is an inlet temperature set in advance.

By virtue of [Configuration 3], the temperature of slurry need not be adjusted after the preparation and temperature increase step but before a step performed following the slurry preparation step (between the preparation and temperature increase device 23 and, for example, the preheater 31). Therefore, the number of devices can be reduced, compared with a case where a device for adjusting the temperature of slurry after the preparation and temperature increase step but before a step performed following the slurry preparation step needs to be provided. As a result, the equipment cost of the equipment (ashless coal production system 1) for performing the ashless coal production method can be reduced.

(Effect 4) [Configuration 4]

In the preparation and temperature increase step (preparation and temperature increase device 23), solvent vapor and slurry is mixed by a venturi scrubber 23 a.

In the venturi scrubber 23 a of [Configuration 4], the mixing of solvent vapor (gas) and slurry (a mixture of solid and liquid) can be unfailingly performed. Therefore, the heat exchange between solvent vapor and slurry can be efficiently performed.

(Effect 5)

The ashless coal production method (ashless coal production system 1) includes a coal feeding step (coal feed line 11) and a vapor discharging step (vapor discharge unit 13). The coal feeding step (coal feed line 11) is a step of feeding coal to be used in the slurry preparation step (to be fed to the slurry preparation device 20) by way of the coal feed line 11.

[Configuration 5]

The vapor discharging step (vapor discharge unit 13) is a step of flowing a purge gas into the coal feed line 11 to thereby discharge the vapor produced in the preparation and dehydration step (preparation and dehydration tank 21) from the coal feed line 11.

By virtue of [Configuration 5], the vapor in the coal feed line 11 can be kept from forming a condensate. Therefore, clogging of the coal feed line 11 due to adhesion of coal to the condensate can be suppressed. In addition, since [Configuration 5] can keep the vapor in the coal feed line 11 from forming a condensate, it is not necessary to prevent the generation of vapor in the preparation and dehydration step (preparation and dehydration tank 21). In turn, the solvent fed to the preparation and dehydration step (preparation and dehydration tank 21) need not be cooled to such a degree that vapor is not generated in the preparation and dehydration step (preparation and dehydration tank 21). Therefore, the number of devices (or the scale of device) can be reduced, compared with a case where a cooler (for example, the cooler 155 of FIG. 2) for performing the cooling above needs to be provided. As a result, the equipment cost of the equipment (ashless coal production system 1) for performing the ashless coal production method can be reduced.

Modification Example

The embodiment above can be variously modified. For example, although the temperature of the solvent or slurry is exemplified in FIG. 1, the temperature of the solvent or slurry may be a temperature different from the temperature exemplified in FIG. 1.

In addition, for example, although the state of solvent (solvent liquid or solvent vapor) is distinguished by a solid arrow and a dot-dash arrow in FIG. 1, the state of solvent may be a state different from the state illustrated in FIG. 1. However, the solvent fed to the preparation and dehydration tank 21 is a solvent liquid, and the solvent fed to the venturi scrubber 23 a is a solvent vapor.

Furthermore, for example, the order of respective steps (the connection order of respective devices) or the presence or absence of each step (each device) may be appropriately changed. [Example 1]: All or part of third circulation path 43, fourth circulation path 44, fifth circulation path 45, sixth circulation path 46, and devices disposed on these circulation paths may not be present. [Example 2]: Although the solvent flowing through the sixth circulation path 46 is fed to the oil/water separation tank 83 after being subjected to heat exchange in the heat exchanger 77, the solvent flowing through the sixth circulation path 46 may be fed to the cooler 81 after being subjected to heat exchange in the heat exchanger 77.

In addition, for example, part or the whole configurations of the ashless coal production system 101 of Comparative Example illustrated in FIG. 2 may be combined with or substituted by part or the whole configurations of the ashless coal production system 1 illustrated in FIG. 1. Specifically, for example, the exhaust heat recovery boiler 193 on the sixth circulation path 46 illustrated in FIG. 2 may be disposed on the sixth circulation path 46 of the ashless coal production system 1 illustrated in FIG. 1.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention. The present application is based on Japanese patent application (Patent Application No. 2013-267439) filed on Dec. 25, 2013 and the contents thereof are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, the heat exchange between coal and solvent and the heat exchange between slurry and solvent are efficiently performed, so that the heat energy generated in the production process of ashless coal can be effectively utilized and thereby the ashless coal can be produced at a low cost.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   1 Ashless coal production system -   11 Coal feed line -   13 Vapor discharge unit -   20 Slurry preparation device -   21 Preparation and dehydration tank -   23 Preparation and temperature increase device -   23 a Venturi scrubber -   33 Extraction tank -   35 Separation unit -   37 Solvent recovery unit -   41 to 46 Circulation path 

1. A method for producing ashless coal, the method comprising: mixing coal and a solvent to prepare a slurry, and performing a dehydration of the coal contained in the slurry and a temperature increase of the slurry; heating the slurry to extract a solvent-soluble component of the coal, to obtain an extracted slurry; separating the extracted slurry into a solution containing the solvent-soluble component of the coal and a solid-content concentrated liquid having concentrated therein a solvent-insoluble component of the coal; evaporating and separating the solvent from the solution to obtain ashless coal; and circulating the solvent to a slurry preparation, wherein the slurry preparation comprises: mixing liquid solvent of the circulating step with the coal to prepare the slurry and dehydrate the coal; and mixing vapor solvent of the circulating step with the slurry to increase the temperature increase of the slurry.
 2. The method according to claim 1, wherein, during the mixing of the vapor solvent with the slurry, a concentration of the slurry is adjusted to be an inlet concentration suitable to extract the solvent-soluble component of the coal.
 3. The method according to claim 1, wherein, during the mixing of the vapor solvent with the slurry, a temperature of the slurry is increased to an inlet temperature suitable for mixing the coal with the solvent.
 4. The method according to claim 1, wherein the vapor solvent of the circulating step and the slurry are mixed by a venturi scrubber.
 5. The method according to claim 1, further comprising: feeding coal, to be mixed with the solvent to prepare the slurry, by way of a coal feed line; and flowing a purge gas into the coal feed line to thereby discharge a vapor produced in the mixing step from the coal feed line.
 6. The method according to claim 2, wherein, during the mixing of the vapor solvent with the slurry, a temperature of the slurry is increased to an inlet temperature suitable for mixing the coal with the solvent. 